As is well understood, there is a close analogy between the reflection and refraction of microwave and of optical wavefronts at boundaries between regions of different refractive index. In the optical field a structure identified as a blazed grating is known; the term "blazed" referring to the fact that the favored response is so much stronger than other responses that the grating appears to be "ablaze", i.e. on fire. Blazed optical gratings are discussed in The Optical Industry and systems Directory-Encyclopedia, Volume II, 1976, The Optical Publishing Co., Inc. Pittsfield, MA. 01202, page E37, and by Jenkins and White, Fundamentals of Optics, McGraw-Hill, 1957, pages 344-347. The design of microwave reflecting gratings employed in the communications and radar systems of the present invention is a special case of the broad class of structures referred to as blazed gratings in the optical field.
In the microwave field, some existing instruments such as frequency division multiplexers and spectrometers employ blazed gratings that function, essentially, to sort different frequencies by directing the multi-frequency input onto a blazed grating for redirection of the individual frequency components along different paths. Functionally, the operation is equivalent to that performed by a multiplechannel wave filter. In the case of the multiplexer, different frequency signals from a common input may be separated by directing the incoming beam onto a blazed grating for redirecting the same onto a multiplicity of receiving means. However, the use of blazed gratings in free space for redirecting microwave energy beams from a radar transmitter or satellite transmitter is not suggested by such microwave multiplexers and spectrometers. A microwave multiplexer employing a blazed grating is disclosed by Henry et al, "A Low-Loss Diffraction Grating Frequency Multiplexer," IEEE Transactions on Microwave Theory and Techniques, Vol. Mtt-26, No. 6, June 1978, pp. 428-433, and a microwave spectrometer is disclosed by Mallory et al, "A simple Grating System for Millimeter and Submillimeter Wavelength Separation", IEEE Transactions on Microwave Theory and Techniques, September 1963, pp. 433-434.
Passive repeaters for use in both space and on the ground are well known as shown, for example, in the following U.S. Patents: Yater, U.S. Pat. No. 3,427,623; Adams, U.S. Pat. No. 3,144,606; and Kreitzberg, U.S. Pat. No. 2,968,033. Some repeaters, such as that shown in Kreitzberg, comprise specular reflectors that are not frequency steerable. Frequency-steerable reflectors, such as those disclosed in the above-mentioned Adams and Yater patents which employ diffraction phenomena, give rise to unsuppressed diffraction lobes which may result in interference in the operation of the associated system, or other systems, and which result in power losses.